Power unit for friction drive hospital bed

ABSTRACT

A friction drive power unit for an electric hi-low hospital bed which unit includes control means accessible to and operable by a patient in the bed to operate the bed in a desired manner. The unit includes an interlock which prevents simultaneous operation of the hi-low and spring moving mechanisms of the unit, and includes limit position control of these devices.

United States Patent Francis J. Burst Batesville, Ind.

Dec. 5, 1968 Mar. 23, 1971 Hill-Rom Company, Inc. Batesville, Ind.

Inventor Appl. No. Filed Patented Assignee POWER UNIT FOR FRICTION DRIVEHOSPITAL BED 8 Claims, 8 Drawing Figs.

[15. Cl 5/67, 74/202 Int. Cl A6lg 7/06, A6 lg 7/00, F16h 15/00 Field ofSearch 5/6669;

[56] References Cited UNITED STATES PATENTS 2,913,300 11/1959 Darnell etal. 5/68 3,198,891 8/1965 Burst et al 5/68 3,246,540 4/1966 Pickles etal 5/68X 3,436,769 4/ 1969 Burst 5/67 Primary ExaminerCasmir A. NunbergAttorneys-George H. Simmons and Edward A. Haight ABSTRACT: A frictiondrive power unit for an electric hi-low hospital bed which unit includescontrol means accessible to and operable by a patient in the bed tooperate the bed in a desired manner. The unit includes an interlockwhich prevents simultaneous operation of the hi-low and spring movingmechanisms of the unit, and includes limit position control of thesedevices.

PAT ENTED m2 319?:

SHEET 1 0 4 g RH W m a a w P a E \Q. wb M AW. ms 4 Qhv b m h d/ llQWElRllll l l'l Mills lFliRllUllllGJN llllhllli illti tii lillhli lEilElllPRIOR ART The present invention is an improvement upon my copendingapplication, her. No. 685,42l, tiled Nov. 24-, i967, now US. Pat. No.3,436,769 issued Apr. 8, 1969. in this prior application a frictiondrive hospital bed includes a lever pivotally mounted upon the foot endof the bed for selecting either the iii-low operating mechanism or thehead section operating mechanism and operation of the selected mechanismis controlled by patient-operated switches that control the electricalcircuit of the reversible electric motor that supplies power to the bed.

SUMMARY GE THE llNVENTlON The present invention improves upon this priorart bed by providing a friction drive power unit which is controlled bya patient in the bed both as to the selection or which of the drivingmechanisms is to be actuated and, simultaneously with this selection,operating an electrical switch which controls the circuit of theelectric motor.

The power unit of the present invention consists of a selfcontainedpower unit that can be assembled almost completely prior to beingmounted in the framework of a hospital bed. The completion of the powerunit after mounting in the framework involves only a minimum of readilyperformable operations. The power unit includes a driving wheelconstantly frictionally engaging a pair of friction discs underpredetermined pressure which pressure is adjustable. The unit contains apair of clutch mechanism each associated with one of the friction discsand operable to establish a driving connection between that disc and anassociated screw shaft. One of said screw shafts controls the hi-lowoperation of the bed and the other screw shaft controls the elevation ofthe head section of the spring frame of the bed.

Operation of the clutches to establish a driving connection is effectedby levers accessible to and operable by a patient in the bed.Simultaneously with the operation of a clutch a switch is operated toclose a circuit to a reversible electric motor to operate the connectedscrew in a direction corresponding to the direction the lever isoperated by the patient. An interlock prevents simultaneous movement ofthe clutches into driving position.

l ower is supplied to the power unit by a separately mounted reversibleelectric motor connected to the unit by a belt located within the crosschannel at the foot end of the bed. The motor mount includes means foradjusting the tension on the belt. Should replacement of the belt becomenecessary, it can be done without disturbing the assembly of the powerunit.

The present invention permits supplying an electrically driven hi-lowbed at a cost sufficiently low to render the bed available for use ininstallations where the features of the more expensive automatichospital beds cannot be justified such as, for instance, in self-caresections of hospitals and in nursing homes and convalescent homes.

The invention will be best understood from the detailed specificationand claims which follow reference being had to the accompanying drawingsin which a preferred embodiment of the invention is shown by way ofexample and in which:

lFlG. l is a plan view of the power unit installed in a hospital bed andwith the dust covers in place;

lFlG. 2 is a side elevational view of the bed shown in HQ. ll;

3 is an end elevational view of the bed drawn to an enlarged scale;

.FlG. is a plan view of the power unit with the mechanisms thereof inneutral position;

PEG. 5 is a fragmentary plan view of the power unit with the iii-lowclutch moved into driving position;

hit is is a cross-sectional view taken substantially along the linetill: of EEG.

Fit 7 is a side elevational view taken substantially along the line 7'7of H6. 5; and

Flt}. it is a schematic diagram of the electrical circuit of the bed.

As will be seen best in FlGS. l and 2, the hospital bed consists of siderails ill and ll which are attached to the cross channel l2 at the headend of the bed. Side rail it flares outwardly and is connected to afootend post and side rail lll extends in a similar manner to a foot endpost lid. Post l3 and i l are connected together by a foot end crosschannel iii to which the power unit of the present invention is mountedand indicated generally at lb.

The details of the hospital bed are not of the essence of the presentinvention and reference is here made to the abovementioned copendingapplication for an explanation of such details. it is sufficient to noteherein that the spring frame of the bed includes a seat section Ell towhich the head section ill of the spring frame is pivotally connected..lournaled in suits ble brackets 22 in the seat section are transversecontrol rods 23 and 24. Fixed upon control rod 23 and dependingtherefrom is a link 25 and affixed upon the control rod 24 is a similarlink as.

As will be seen best in FiGS. d to 55, inclusive, the power unit of thepresent invention consists of a framework indicated generally at 363 andconsisting of a mounting bracket Lil having end flanges 32 from whichare projected cars 33 which are pierced to permit attaching the bracketto the web of the cross channel i5 in convenient manner such as by bolts34.

Fixed to the bracket 3i on the side thereof away from the channel 15 arescrew channels 35 and 3b which from FlG. s will be seen to berectangular in cross section and to have an open top. These channels areattached to the bracket in con venient manner such as by welding. Fixedbetween the channels 35 and 3b is a base plate 37 located adjacent themounting bracket Ill. Plate I37 is attached to the channels inconvenient manner such as by welding. Also extending between thechannels 35 and 3d and located adjacent the distal ends of thesechannels is a tie plate Bil which is attached to the chan nels inconvenient manner such as by welding. Plate 35* contains a downwardlyextending flange 39 which is attached to the flange of a crossmember H7in convenient manner such as by bolts to complete the mounting of theframework Bil in the underframe of the bed.

A hi-low screw 45 is journaled in bearings ll and 4i? that are fixed inthe channel 35. The hi-low nut th is threaded upon shaft 3$ and two pairof cables 59 and fill and 52 and 53 are associated with the nut 43 andattached thereto in known manner.

As will be seen in H6. ll, cables dd and 5E extend through suitablesheaves on the cross channel M at the head end of the bed to the legstelescoped in the posts at the head end of the bed and cables 52 and 53extend around sheaves 54 supported in the crossmember lid of the frameand thence to posts l3 and M respectively at the foot end of the bed tosupport the underframe and spring frame of the bed on the legstelcscoped into the posts in known manner.

Mounted upon the shaft 45 is a clutch element SE having a hub 56 thatencircles the shaft and is prevented from rotation thereon by suitablemeans such as a pin fixed in the shaft and extending through alongitudinal slot in the hub. A friction disc shaft 57 is joumaled inthe channel 1% by a bearing 5% with the axis of the shaft 57 coincidingwith the axis of the shaft 45. Shaft 5'7 carries diametrically opposedpins $9 for a purpose hereinafter to be explained. Shaft E projectsthrough a perforation in the mounting bracket 31 and a friction disc tois fixed upon this shaft and positioned between the bracket Bi l and theweb of channel l5. A friction facing til is tired upon the periphery ofthe disc till.

1' he head screw s2 is journaled in suitable bearings di l and se thatare fixed in the screw channel 1% and a nut is threaded upon this shaft.Links as connected to the nut 65 are extended to the head section of thebed and function to elevate that head section as the nut 65 is movedaway from the foot end of the bed.

The clutch element s7 is supported upon the shaft 62 by a hub 61 whichis prevented from rotation upon the shaft by suitable means such as apin carried by the shaft and projecting through an elongated slot in thehub.

A friction disc shaft on is journaled in a suitable bearing 71) that isfixed in the screw channel Sowhich bearing supports the shaft with itsaxis coinciding with the axis of the shaft 62. Shaft 69 carriesdiametrically opposed pins 71. Shaft 69 projects through a perforationin the mounting bracket 31 and a friction disc 72 is fixed upon thisshaft and positioned between the bracket 31 and the web of flange 15. Afriction facing material 73 is attached to the disc 72.

As will be seen in F168. 4 and '7, base plate 37 contains an upstandingflange S11 adjacent the mounting bracket 31. Projecting from this flangeare spaced-apart bearing ears 81 and in which a bearing is supported forlimited movement around a horizontal axis. As will be seen in 1 16. 7,an L- shaped plate E54 is attached to the web of the cross channel 15 bystuds projecting through vertically disposed slots in the plate.Attached to this plate is a bearing 55. A shaft 86 is journaled inbearings 82 and 55 and projects into the channel 15. A pulley wheel 87is attached to the end of shaft as. A friction drive wheel is mountedupon the shaft 86 between bearings 52 and 85.

As will be seen best in F165. 1 and 3, a motor-mounting plate 51) isattached to the web of the cross channel 15. The base 2 of an electricmotor 91 is supported upon this plate by studs that project throughslots in the base. The shaft 5 3 of the motor 92 projects through aperforation in the web of channel and a motor pulley 94 is fixed uponthe end of shaft 93. A belt 95 encircles the pulley wheels 57 and 94 toestablish a driving connection therebetween. To maintain a desiredtension upon the belt a tension adjusting device 96 is mounted upon themotor mount 91 and bears against the motor base 92.

Friction drive wheel 88 engages the friction facings on the two discs todrive the same as the drive wheel is rotated. As will be seen best inFIG. 3, the axis of the shaft 86 is located slightly above the axes offriction disc shafts 5'7 and 69. To maintain a desired pressure betweenthe driving wheel 88 and the discs a bracket 11111 is fixed upon thechannel 15 and a counterbored block 1111 is attached thereto. Anadjusting screw 102 is threaded through the bracket 101) and block 101and bears against a compression spring 1113 which bears against theupper ann of the L-shaped bracket 14 through a suitable bearing button.By rotating the screw 102 the spring 103 may be compressed to increasethe pressure between the driving wheel and friction discs. In thismanner, a predetermined pressure can be maintained and slippage of thedriving wheel upon the discs thereby prevented.

Fixed upon base plate 37 in suitable manner such as by welding are pivotbosses 105 and 1116.11 pivot pin 107 projects into the boss 1115 and isattached to a clutch arm 108 in convenient manner such as by welding. Aboss 109 is attached to the distal end of arm 1%.

A yoke pin 110 is journaled in the boss 109 and attached to yoke 111.Mounted upon the hub 56 of the clutch mechanism is a spool 112containing a groove into which the yoke 111 is fitted. A spring 113encircles the hub 56 and bears against the spool 112 and element 55 ofthe clutch.

in a similar manner, a pivot pin 114 is joumaled in boss 1% and attachedto clutch arm 115 upon the distal end of which a boss 116 is fixed. Ayoke pin 1.17 is journaled in boss 116 and attached to yoke 115 whichfits into a groove in a spool 1119 on hub 68 of the clutch assembly. Aspring 121 encircles this hub and bears against the spool 119 and aclutch element 67.

Normally spool is held against a snap ring 122 that is fitted in agroove hub 56 by spring 113 and spool 119 is held against a snap ring12.3 that is fitted in a groove in hub 68 by spring 121.

To provide for control of the bed by a patient therein, 1 have mounted ahi-low control rod 125 in brackets 12s fixed upon the screw channel 35.Control rod 125 is preferably rectanguthe lar in cross section andbushings 126 are preferably formed from a low resistance plastic such asnylon. The link 12'] connects control rod to link 25 on the hi-lowtransverse rod 23. it will be apparent that rotation of rod 23 in onedirection moves the control rod 125 longitudinally in a correspondingdirection and vice versa.

Fixed upon the end of control rod 125 is an actuator plate 128 whichcontains an car 129 that bears against the clutch arsn 1115 at a pointbenveen the pivot pin 127 and the boss 1119. To this arrangementmovement of the control rod 125 towards the foot end of the bed rotatesthe actuator arm to move the spool 112 towards the clutch element 55thereby compressing the spring 113. When the pins 59 on the shaft 57register with end grooves in clutch element 55, the element moves intoengagement with these pins to establish a driving connection betweenfriction disc 59 and screw shaft 45. Also mounted upon the actuatorplate 128 is a pin 130 which engages the clutch arm 10% at a point onthe side of the pivot pin 11 7 opposite the boss 1115. Through thisarrangement movement of the control rod away from the foot of the bedrotates the clutch arm 1118 to tension spring 113 in the foregoingmanner.

To provide for control of the head section of the spring frame, I havemounted a control rod 1.31 in bushings 132 which are fixed upon thescrew channel 36. A link 133 connects rod 131 to the link 126 on thehead control transverse rod 24. Fixed upon the end of control rod 131 isan actuator plate 134 which has an car 135 that bears against the clutcharm 115 between the pivotal support of that arm and the boss 11o carriedthereby and the plate 134 also has a pin 136 which engages the plate 115on the side of the pivotal support of that plate opposite thatcontaining the boss 1111. Through this arrangement movement of thecontrol rod 131 in either direction operates the clutch in the abovemanner.

The electrical circuit of the motor is controlled by switches 138, 139,143 and 1 Preferably, these switches are momentary snap actionsingle-pole switches of which a number are available on the open market.Arm 140 of switch 138 and arm 141 of switch 138 extend to opposite sidesof a earn 137 carried upon the control rod 125. In a similar manner. armof switch 143 and arm 146 of switch 144 are disposed upon opposite sidesof a cam 142 carried upon a control rod 131.

As will be seen in FIG. 3, the mainsprings of switches 138, 159, 143 and144 are connected together and connected to line L2 of a source ofcommercial power. Make contacts of switches 138 and 143 are connectedtogether and to a junction of a motor winding and condenser 152. Motorwind ing 151 is connected to winding 1511 and the other line L! of thesource of commercial current is connected to this junction. A secondcondenser 153 is connected to condenser 152 and is normally bypassed bya centrifugal switch 1154. Through this arrangement, closing of eitherswitch 138 or 133 extends line L2 through winding 150 to line L1 with anauxiliary circuit extending through condenser 152 and centrifugal switch154 and thence through the winding 151 to L1. The winding 159 is thusthe main winding and the winding 151 the auxiliary winding and the motoroperates in one direction. As soon as the motor gets up the speed,centrifugal switch 15% opens to connect condenser 153 in series withcondenser 152.

When either switch 139 or 144 is operated, line L2 is connected throughwinding 151 to line L1 with an auxiliary circuit through the centrifugalswitch 154 and condenser 152 and winding 1511 to L1. The motor operatesin the opposite direction.

To aid in the holding the control rod 125 in operated posi tion during ahi-low operation of the bed. l have provided an arm 1% that is mountedon the bearing 4.; and carries at its distal end a spring pressed button157 which engages indentations 15d. HQ. 5, in each of the threepositions in which the control arm may be located.

Limit positions of the bed are determined and maintained in the normalmanner. when control rod is moved towards the foot end of the bed, nutis moved towards the head end of the bed to lower the bed on itssupporting posts. A bar 160 carried on nut 48 is moved into engagementwith a pin 159 on control rod 125 and the nut moves the rod back tonormal to stop the motor. When the nut 48 is moving towards the foot endof the bed bar 160 engages pin 161 to stop the motor. In a similarmanner bar 163 on nut 65 engages pins 162 and 164 on control rod 131 toestablish limit positions.

Operation of the limit position control, in addition to stopping themotor, moves the spool 112, from the position shown in FIG. 4, intoengagement with snap ring 122. Compression of spring 113 is thusrelieved but the clutch element is not moved, rather it remains indriving engagement with pins 59. Thus, it will be seen that the drivingconnection established by a clutch remains intact upon restoration ofthe control rod that established the connection.

To prevent establishment of simultaneous driving connections aninterlock is provided. As will be seen best in FIGS. 6 and 7, a mountingear 165 extends from the top of flange 80 and is disposed parallel tobase plate 37. Pivotally mounted upon car 165 is an interlock bar 166one end of which terminates adjacent boss 109 on yoke 111 and the otherend terminates adjacent boss 116 on yoke 118.

When a patient in the bed operates control rod 131 to move yoke 118 awayfrom snap ring 123, bar 166 is rotated on its pivot to move clutchelement 55 out of engagement with pins 59 prior to movement of clutchelement 67 into engagement with pins 71. When clutch element 67 is indriving engagement with pins 71, operation of hi-low control rod 125causes interlock bar 166 to rotate in the opposite direction to moveelement 67 out of engagement with'pins 71 before element 55 engages pins59. The establishment of simultaneous driving connections is thusdefinitely prevented.

To guard against the entrance of lint and other extraneous matter intothe power unit, a dust cover 124, FIGS. 1 and 2, is provided. This coverincludes a portion covering the motor. The cross channels at the headand foot ends of the bed are equipped with the usual covers to completethe protection of the power mechanism.

From the foregoing, it will be apparent that the power unit of thepresent invention possesses many advantages. Since the power unit is aself-contained structure, it can be assembled on a bench, and the unitthen secured in the frame of the bed in a convenient manner. Theassembly is completed by simple operations that can be performed at lowcost. As a result, the cost of the bed is low enough to render itavailable in installations where manually operated beds have been usedheretofore. Since selection of the operation to be performed and thestarting of that operation are made by the patient in the bed, lessattention to the patient by the hospital attendants is required.

The power unit is capable of operating for a long period of time withoutmaintenance and should replacement of the belt become necessary, thiscan be done without disturbing the main assembly of the unit.

I claim:

1. A power unit for a hi-low hospital bed that has a spring frame whichincludes a seat section to which a head section is pivotally connected,has a pair of transverse control rods journaled in said seat section andavailable to a patient in the bed and has an underframe that includes across channel at the foot end of the bed, said unit comprising:

a. a power unit framework consisting of a mounting bracket fixed uponsaid cross channel and having a center section spaced therefrom, a pairof spaced-apart screw channels fixed to said mounting plate on the sidethereof away from said cross channel, a base plate fixed between saidscrew channels adjacent said mounting plate, and a tie plate fixedbetween said screw channels adjacent the distal ends thereof;

b. a screw shaft joumaled in each screw channel by bearings fixed in thechannel;

cT'a friction disc associated with each screw shaft positioned betweenthe mounting bracket and the cross channel and fixed upon a shaftjoumaled in the screw channel in axial alignment with the screw shaft;

d. a drive wheel frictionally engaging both of said friction discs isfixed upon a drive shaft that is joumaled in a first bearing supportedon the base plate and in a second bearing supporting upon the web of thecross channel through which web the shaft extends;

e. an electric motor mounted on a motor mount fixed upon the web of thecross channel with the motor shaft extending through a perforation inthat web;

f. a belt encircling sheaves fixed on the motor and drive shafts toestablish a driving connection for the drive wheel; I

g. clutch means including a pair of clutch mechanisms for establishingdriving connections between said discs and screw shafts;

h. means operable by a patient in the bed for operating said clutchesinto driving positions; and

i. an interlock for preventing simultaneous movement of said clutchmechanisms into driving positions.

2. A power unit as specified in claim 1 in which said first bearing issupported for limited movement around a horizontal axis and the secondbearing is mounted upon an L-shaped plate mounted upon the web of thecross channel for limited vertical movement and in which a springpressed button engages the L-plate and urges and maintains the drivewheel against the friction discs under predetermined pressure.

3. A power unit as specified in claim 1 in which each clutch mechanismincludes diametrically opposed pins on the end of said friction discshaft, a clutch element having a hub telescoped over the screw shaft andheld against rotation thereon while capable of axial movements thereon,said clutch element containing radial slots into which said pins arepositioned by axial movement of the element to establish a drivingconnection between the friction disc and screw shaft.

4. A power unit as specified in claim 3 in which there is a clutch armpivotally supported upon the base plate, a boss fixed upon said arm, ayoke, a pin fixed upon said yoke and joumaled in said boss, a spoolpositioned upon the hub of the clutch element and containing a grooveinto which said yoke fits, a spring encircling said hub and engagingsaid element and said spool, rotation of said arm moving said spoolaxially.

on the hub to compress said spring and thereby move said clutch elementinto driving engagement with said pins.

5. A power unit as specified in claim 4 in which the patient operablemeans includes a pair of longitudinal control rods supported in saidframework for longitudinal movements, link means connecting eachlongitudinal rod to one of said transverse rods, and clutch-operatingmeans fixed upon each longitudinal rod.

6. A power unit as specified in claim 5 in which the clutchoperatingmeans includes an actuator plate fixed upon the control rod and engagingthe clutch arm at a point between the pivotal support of that arm andthe boss on said arm, said actuator plate having a pin that engages theclutch arm at a point on the arm on the side of the pivotal support ofthe arm opposite the point of engagement of the plate with that arm, sothat movement of the control rod and actuator plate from neutral ineither direction moves the clutch element into driving engagement withsaid pins.

7. A power unit as specified in claim 5 in which a pair of momentarysingle-pole snap switches are associated with each longitudinal controlrod and in which a cam fixed upon the rod is positioned midway betweenthe operating arms of said switches when the rod is in neutral position,said cam operating one of said switches as the rod is moved from neutralin one direction and operating the other one of said switches as the rodis moved from neutral in the opposite direction.

8. A power unit as specified in claim 4 in which the interlock includesa bar pivotally mounted upon said base plate and extending intojuxtaposition to said clutch arm bosses, said bar being rotated on itspivot by movement of one of said clutch mechanisms into drivingengagement with the pins associated with it, to move the other clutchmechanism out of engagement with the pins associated with it.

1. A power unit for a hi-low hospital bed that has a spring frame whichincludes a seat section to which a head section is pivotally connected,has a pair of transverse control rods journaled in said seat section andavailable to a patient in the bed and has an underframe that includes across channel at the foot end of the bed, said unit comprising: a. apower unit framework consisting of a mounting bracket fixed upon saidcross channel and having a center section spaced therefrom, a pair ofspaced-apart screw channels fixed to said mounting plate on the sidethereof away from said cross channel, a base plate fixed between saidscrew channels adjacent said mounting plate, and a tie plate fixedbetween said screw channels adjacent the distal ends thereof; b. a screwshaft journaled in each screw channel by bearings fixed in the channel;c. a friction disc associated with each screw shaft positioned betweenthe mounting bracket and the cross channel and fixed upon a shaftjournaled in the screw channel in axial alignment with the screw shaft;d. a drive wheel frictionally engaging both of said friction discs isfixed upon a drive shaft that is journaled in a first bearing supportedon the base plate and in a second bearing supporting upon the web of thecross channel through which web the shaft extends; e. an electric motormounted on a motor mount fixed upon the web of the cross channel withthe motor shaft extending through a perforation in that web; f. a beltencircling sheaves fixed on the motor and drive shafts to establish adriving connection for the drive wheel; g. clutch means including a pairof clutch mechanisms for establishing driving connections between saiddiscs and screw shafts; h. means operable by a patient in the bed foroperating said clutches into driving positions; and i. an interlock forpreventing simultaneous movement of said clutch mechanisms into drivingpositions.
 2. A power unit as specified in claim 1 in which said firstbearing is supported for limited movement around a horizontal axis andthe second bearing is mounted upon an L-shaped plate mounted upon theweb of the cross channel for limited vertical movement and in which aspring pressed button engages the L-plate and urges and maintains thedrive wheel against the friction discs under predetermined pressure. 3.A power unit as specified in claim 1 in which each clutch mechanismincludes diametrically opposed pins on the end of said friction discshaft, a clutch element having a hub telescoped over the screw shaft andheld against rotation thereon while capable of axial movements thereon,said clutch element containing radial slots into which said pins arepositioned by axial movement of the element to establish a drivingconnection between the friction disc and screw shaft.
 4. A power unit asspecified in claim 3 in which there is a clutch arm pivotally supportedupon the base plate, a boss fixed upon said arm, a yoke, a pin fixedupon said yoke and journaled in said boss, a spool positioned upon thehub of the clutch element and containing a groove into which said yokefits, a spring encircling said hub and engaging said element and saidspool, rotation of said arm moving said spool axially on the hub tocompress said spring and thereby move said clutch element into drivingengagement with said pins.
 5. A power unit as specified in claim 4 inwhich the patient operable means includes a pair of longitudinal controlrods supported in said framework for longitudinal movEments, link meansconnecting each longitudinal rod to one of said transverse rods, andclutch-operating means fixed upon each longitudinal rod.
 6. A power unitas specified in claim 5 in which the clutch-operating means includes anactuator plate fixed upon the control rod and engaging the clutch arm ata point between the pivotal support of that arm and the boss on saidarm, said actuator plate having a pin that engages the clutch arm at apoint on the arm on the side of the pivotal support of the arm oppositethe point of engagement of the plate with that arm, so that movement ofthe control rod and actuator plate from neutral in either directionmoves the clutch element into driving engagement with said pins.
 7. Apower unit as specified in claim 5 in which a pair of momentarysingle-pole snap switches are associated with each longitudinal controlrod and in which a cam fixed upon the rod is positioned midway betweenthe operating arms of said switches when the rod is in neutral position,said cam operating one of said switches as the rod is moved from neutralin one direction and operating the other one of said switches as the rodis moved from neutral in the opposite direction.
 8. A power unit asspecified in claim 4 in which the interlock includes a bar pivotallymounted upon said base plate and extending into juxtaposition to saidclutch arm bosses, said bar being rotated on its pivot by movement ofone of said clutch mechanisms into driving engagement with the pinsassociated with it, to move the other clutch mechanism out of engagementwith the pins associated with it.